The present invention is directed to a process of lamination of photopolymerizable solder mask layer onto a printed circuit substrate containing a number of holes wherein a liquid is present during application of the layer to the substrate.
Related lamination processes are known in the prior art but each are considered defective when a relatively thin solder mask film is employed. Defects are introduced into the liquid/solder mask film in the area above holes in the substrate. Such defects are conventionally in the form of cracks which propagate into defects in later processing such as in application of molten solder.
A commercial process for lamination of liquid/solder mask film onto a printed board substrate is marketed by E. I. du Pont de Nemours and Company under a trademark, Valu.RTM. solder mask lamination. However problems exist due to defects which occur in the film/liquid which is above holes in a substrate.
These defects are particularly troublesome for thinner films as 2.0 mils or less and particularly for 1.5 miles or less and for high aspects hole, i.e., when the ratio of the height of the hole to its diameter is at least 4 and more specifically at least 5.
U.S. Pat. No. 4,966,827 discloses various embodiments of obtaining printed wiring boards by means of a two layer composite coating. In one embodiment an adhesive photopolymer layer is applied to a printed wiring board in a liquid state displacing air from the printed wiring board surface. A dry film solder mask is temporarily adhered to the underside of a screen printing frame and applied onto the liquid layer prior to processing of the photopolymer layer. In another embodiment a layer of curable, 100 percent solids liquid of predetermined thickness is employed as the adhesive layer.
European Patent Application 87113176.9, discloses a process of laminating a photopolymerizable film which is preformed and nonliquid onto a substrate surface employing a photosensitive liquid of at least one monomer and a photoinitiator.
When a film/liquid lamination process of the prior art is employed to apply a thin solder mask of a thickness not greater than 2.0 mil to circuit boards containing high aspect ratio holes, an undesirable number of tent failures (i.e., failures above or immediately surrounding a hole on the film surface) have been observed after thermal curing and/or soldering wherein cracks are formed in the surface of the tent above the barrel of the hole. Such surface cracks may extend to the underlying circuit pad or they may be micro cracks confined to the surface area of the solder mask. In either case, the cracks absorb ionic contaminants during further processing which cannot be easily removed and which adversely impact the circuit board function. The number of tenting failures is further exacerbated when the thickness of the dry film is reduced, e.g., for use with surface mount technology.
There is an increasing industry need to use thinner solder mask layers to be compatible with the emerging surface mount technology and there is also a need to use such technology on high aspect ratio multilayer printed circuit boards having defect free tented or plugged holes.